Cartridge fuses are often included in circuits for the protection of electrical equipment, for example each phase, including the neutral conductor, of an electrical circuit, can include a cartridge fuse. Such a fuse may be carried in a fuse holder which is an independent unit connected into the circuit to be protected, or in a fuse holder which is combined with for example an electrical switch. In the latter case the fuse may be inserted into a suitable recess or housing provided in the insulating casing of the switch, which may be manual or automatic.
It has been proposed that a cartridge fuse can be secured in the housing by a ring nut which is screw-threadedly fitted onto a screw-threaded ring at the mouth of the housing, the ring nut being tightened on the ring to provide a locking action to secure the fuse in the housing. Tightening of the ring nut also causes a compression force to be applied to the cartridge fuse in the axial direction thereof, to ensure electrical contact and continuity. However, the ring nut is not always capable of maintaining a secure contact with the correct degree of compression of the cartridge fuse, since the ring nut may tend to become slackened. In addition, the production and fitting of a ring nut and a companion screw-threaded ring generally involve substantial cost.